It looks like they've found protonium in the decay of a heavy particle! 🎉
Protonium is made of a proton and an antiproton orbiting each other. It lasts a very short time before they annihilate each other.
It's a bit like a hydrogen atom where the electron has been replaced with an antiproton! But unlike a hydrogen atom, which is held together by the electric force, protonium is mainly held together by the strong nuclear force. It's also much smaller than a hydrogen atom.
There are various ways to make protonium. One is to make a bunch of antiprotons and mix them with protons. This was done accidentally in 2002 during the first experiment that created antihydrogen. They only realized this upon carefully analyzing the data 4 years later.
This time, people were studying the decay of the J/psi particle. The J/psi is made of a heavy quark and its antiparticle. It's 3.3 times as heavy as a proton, so it's theoretically able to decay into protonium. And careful study showed that yes, it does this sometimes!
The new paper on this has over 550 authors, so I won't list them all. It also has a rather dry title - not "We found protonium!"
The idea here is that sometimes the J/ψ particle decays into a gamma ray and 3 pion-antipion pairs. When they examined this decay, they found evidence that an intermediate step involved a particle of mass 1880 MeV/c², a bit more than an already known intermediate of mass 1840 MeV/c².
This new particle is a bit lighter than twice the mass of a proton, 938 MeV/c². So, there's a good chance that it's protonium!
@johncarlosbaez
Dumb question: by "orbiting" does that mean they're circling each other Newtonian style, or existing in a probability cloud, electron style?
@stib - things this small are very quantum mechanical so "orbiting" should make you think of "orbitals", i.e. probability clouds of certain shapes, not Newtonian orbits. In physics we often talk classically but think quantum-mechanically.
The picture I used for my post is also sort of misleading, because it looks like the proton and antiproton are moving in well-defined circles, which is far from true... though it shows a vague probability cloud in the background.
@johncarlosbaez ...and not to be too niggly at the hairy edge of detectability, but 2×938M𝑒V=1876M𝑒V, so 1882M𝑒V would be a little h𝑒𝑎𝑣𝑖𝑒𝑟 than 2 isolated protons which is not what I would naively expect...
@johncarlosbaez Adults often would like to ask questions but fear to embarass themselves.😉 I'm sure that I would like to ask stupid (no: curious) questions but I have to read your text several times to find them.
It sounds fascinating but I don't understand everything. @lightninhopkins
@johncarlosbaez@NatureMC@lightninhopkins
On your wordpress Will said "A word of caution is that that what they are doing here (trying to disentangle overlapping broad resonances in a line shape) is very difficult."
If anything, Will is understating it; the difficulty is 100% general, not limited to particle physics; it's notorious throughout the signal processing world that such separations are historically extremely fraught and often wrong and often depend far too much on heuristics (1/e^2 for instance is usually a rule of thumb, not a hill to die on).
The first and foremost place I've seen this carefully analyzed is in mathematical analysis of effective resolution of microscopy of various forms, but peak separation of course is a totally general problem.
BTW another form of this, far from particles, but that affects e.g. the cell phones in our pockets:
These days the state of the art in semiconductor manufacturing feature size exceeded the diffraction limit long ago, which used to be regarded as impossible, but companies like TSMC (Taiwan Semiconductor Manufacturing Company) pre-distort the mask image to compensate -- along with a large number of trade secret hacks.
Otherwise they'd have to be using gamma ray illumination of masks, which turns out to be highly problematic.
@johncarlosbaez The J/psi is itself famous: more than any other single particle, it was the one whose discovery really put the general theoretical framework we now know as the Standard Model of Particle Physics on solid ground.
And I like the "as above, so below" quality of it, that the J/psi is itself a bound state of a particle and its antiparticle. Its peculiar double name comes from its having been discovered pretty much simultaneously by two teams that gave it different names--no point in arguing over scientific priority there.
@johncarlosbaez The J/psi is longer-lived than you would expect. It's a bound state of a charm quark and an anti-charm quark. These can annihilate each other by turning into photons, like positronium. But since they're strongly interacting particles, they could also annihilate into gluons, the quanta of the "color" force in quantum chromodynamics. And then those gluons could immediately decay into some other stuff.
That does happen, but it happens less often than you might expect. That's apparently an example of a general pattern called the "OZI rule", that any high-energy strong-interaction process where the diagram is made of pieces connected by just gluons is less probable than you'd naively expect. One way to justify that is asymptotic freedom, the way the strong-interaction coupling tends to get weaker when the energies exchanged are large--if all the energy in the interaction is going through the gluons, that's pushing the interaction into that regime.
@jannem - There's a game called Robocraft where you try to destroy your enemy's "protonium reactors". These are imaginary devices powered by imaginary "protonium crystals".
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